Pilot flow pressure control valves are extensively used for controlling the pressure at which a larger control valve will open to relieve or control the pressure in a hydraulic system. Normally, such pilot valves are comprised of a valve element having a surface or surfaces exposed to the hydraulic pressure to be controlled which creates a pressure force tending to open the valve. A compression spring biases the valve element to the closed position. When the pressure force exceeds the spring force or bias, the valve opens to allow the flow of fluid therethrough. This fluid flow then changes pressure forces on the larger valve allowing it to open and prevent further increases in the fluid pressure.
To determine the opening pressure, the amount of spring compression of bias force on the valve element is initially adjusted at the time of manufacture or is controlled in the field by means of a threaded member which is rotated either manually or by an electric motor.
The use of springs to determine the opening pressure of the valve presents three problems. First, manual adjustment prevents remote control and electric motor adjustment is expensive, bulky, relatively slow to react and requires relatively large amounts of pulsed electrical power. Computer control of such valves in industrial processes is difficult.
Secondly, springs inherently give to the valve a rising pressure versus flow characteristic. Thus, as the valve opens to permit flow, the spring is compressed. As the spring is compressed, its bias force increases in direct proportion to the amount of compression. A progressively higher fluid force or fluid pressure is required to open the valve further and further. For example, with a spring having a force versus compression rate (hereinafter called "spring rate") of 30 pounds per inch, to fully open a pressure control valve 0.020 inches, requires an increase in force of 0.60 pounds. With a 0.030 inch diameter valve seat, this calculates to a pressure increase of 42.4 pounds per square inch (psi) to half open the valve and 84.8 psi to fully open the valve. This results in a rising pressure versus flow characteristic particularly at the smaller valve openings which is considered undesirable for accurate pressure control.
Thirdly, unless an expensive and complicated compound wound spring is employed, it is almost impossible to provide a spring-biased, pilot relief valve which will perform equally well at both high and low pressures. Thus, at low pressures e.g. 100 psi, the valve must open the maximum designed amount to permit the necessary fluid flow to effect operation of the main relief valve. At the higher pressures, e.g. 6,000 psi, the valve need only open a very small amount to permit the necessary volume of fluid flow to actuate the main relief valve. Thus, if a low spring rate spring is employed to give good pressure versus flow characteristics at low pressures, when the spring is compressed to require a higher opening pressure, the valve becomes unstable.
If a high spring rate spring is employed to give stability and good pressure versus flow characteristic at higher pressure, then the pressure versus flow characteristics at low pressure are poor.
At the higher pressures, this adverse effect of a rising force-displacement characteristic of a spring, is acerbated by the apparent drop in pressure on the valve element as the valve opens due to the conversion of the pressure energy on the element to velocity energy. This effect will hereinafter be referred to as the "Bernoulli effect." The Bernoulli effect results in an apparent drop of up to about 10% in the opening pressure force on the valve element as the flow increases. This apparent lowering of the pressure force against the increasing spring bias further adversely effects the pressure versus flow characteristics of the valve.
It is known to provide electro-magnetically controlled valves employing solenoids. However, heretofore the electro-magnetic force-displacement curves of the solenoid were such as to make them unsatisfactory for use in place of a variably compressed mechanical spring.